1. Field of the Invention
The present invention relates to an apparatus and method for controlling an array antenna for use in communications, and in particular, to an apparatus and method for controlling an array antenna comprising a plurality of antenna elements with improved incoming beam tracking.
2. Description of the Related Art
There has been produced on trial a phased array antenna for use in satellite communications that is installed in a vehicle or the like and automatically tracks the direction of a geostationary satellite by Communications Research Laboratory of Japanese Ministry of Posts and Telecommunications, wherein the phase array antenna is referred to as the first prior art hereinafter. The phased array antenna of the first prior art is comprised of nineteen microstrip antenna elements, and is equipped with a total of eighteen microwave phase shifters each provided for each element except for one element so as to electrically scan the direction of a beam without any mechanical drive. In this case, there is provided a magnetic sensor that detects the direction of geomagnetism and calculates the direction of the geostationary satellite when seen from a vehicle, of which position has been previously known, serving as a sensor for controlling the directivity of the antenna and tracking the direction of an incoming beam as well as an optical fiber gyro that detects a rotational angular velocity of the vehicle and constantly keeps the direction of the beam with high accuracy. By combining these two sensors, the antenna directivity is directed to a predetermined direction regardless of the presence or absence of an incoming beam, so that the directivity is always kept constantly in an identical direction even when the vehicle moves.
Furthermore, for a digital beam forming antenna for satellite communication using a digital phase modulation, a phase detection method for acquiring and tracking the incoming beam has been proposed by the present applicant, wherein the phase detection method is referred to as the second prior art hereinafter. The second prior art method is a method implemented by providing a carrier wave regenerating circuit employing a costas loop for each antenna element of an array antenna, controlling the phase of a voltage controlled oscillator (VCO) so that all the elements are put in phase, and then obtaining an array output through in-phase combining of the resulting signals. Further, according to the above-mentioned method, a phase uncertainty takes place at each antenna element in the carrier wave regenerating circuit, and consequently a great amount of power loss occurs when the signals are combined as they are. Therefore, a pull-in phase is detected from a baseband output of each antenna element, and a phase correction amount is calculated based on the detected pull-in phase, so that the phase uncertainty is corrected by a phase shifter prior to the above-mentioned in-phase combining process. According to the second prior art method, the directivity of the antenna is automatically directed to the incoming beam so long as a signal to be received is a phase-modulated wave, and therefore, no special sensor is required for perceiving the direction of the incoming beam.
In the case of the phased array antenna of the first prior art, a magnetic sensor capable of detecting an absolute azimuth is used for directing the directivity of the antenna toward the satellite. However, in the case of a vehicle or the like, the body thereof is made of metal and is often magnetized, and this causes an error in the direction of the directivity of the antenna. In order to eliminate the above-mentioned problems, it is necessary to perform a calibration with magnetic data obtained by rotating the antenna by 360 degrees in a broad place free of any magnetized structure and so forth. Even though the calibration is effected satisfactorily for the achievement of acquiring and tracking of the direction of the satellite, the geomagnetism is often disturbed by surrounding buildings, the other vehicles and so forth, and therefore, it is difficult to track the direction of the incoming beam only by means of the magnetic sensor. For the above-mentioned reasons, the tracking is performed principally based on data obtained from the optical fiber gyro after the direction of the satellite is acquired. However, the optical fiber gyro detects only the angular velocity, not the absolute azimuth as performed by the magnetic sensor, and therefore, azimuth angle errors accumulate. In order to eliminate this problem, there is adopted a method of calibrating in a predetermined period the optical fiber gyro based on information obtained from the magnetic sensor, however, the control algorithm therefor becomes complicated, and also no highly accurate control algorithm has been developed yet.
The phased array antenna of the first prior art has another drawback that, though the beam can be directed in the direction of a signal source when the direction of the signal source has been already known regardless of the presence or absence of the incoming beam, when the direction of the signal source has been unknown or the signal source itself moves as in the case of a satellite in a low-altitude earth orbit, the satellite cannot be tracked except for a case where the movement thereof can be estimated. As described above, the acquiring and tracking method utilizing an azimuth sensor has had such a problem that it has a complicated structure and limited capabilities.
Furthermore, in the case of the phase detection method of the second prior art, a directivity is formed by regenerating a carrier wave for each antenna element. Therefore, the above-mentioned method has the advantageous feature that it requires neither an azimuth sensor as provided for the phased array antenna of the first prior art nor a complicated control algorithm. However, the carrier wave regenerating circuit employs a costas loop circuit for effecting phase-synchronized tracking in a closed loop, and this causes a problem that a certain time is required in achieving convergence in an initial stage of acquiring the incoming beam. In particular, when satellite communication is carried out with the antenna installed in a mobile body such as a vehicle, signal interruption frequently occurs due to trees, buildings and so forth, and therefore, the initial acquisition must be performed speedily within several symbols of received data.
The phase detection method of the second prior art has another problem that a received signal-to-noise power ratio per antenna element is reduced when the array antenna has a great number of antenna elements, and therefore, a phase cycle slip occurs at each antenna element, consequently resulting in difficulties in regenerating a carrier wave and utilizing the gain of the array antenna.